Automotive and industrial automatic transmissions and the like comprise clutch pack assemblies, sometimes also called multi-disc clutch packs. Clutch pack assemblies comprise frictional members, commonly called plates. There are typically two groups of frictional members. A first group comprises plates having a friction enhancing material bonded to the plates, the first group of plates being called friction plates. The second group of plates separates the friction plates and are typically called separator or reaction plates. The separator plates act as reaction members against which the friction plates operate in transferring torque. Typically, one group of plates is internally splined to a first shaft while the other set of plates is externally splined either to a second shaft or a fixed housing as will be discussed more fully below. There will typically be found several clutch pack assemblies in an automatic transmission. Each clutch pack assembly may be selectively used to provide a particular transmission ratio, better known as first gear, second gear, and so on.
To operate effectively, the plates must exhibit high torque transmitting capacity. High torque transmission is related to the coefficient of static friction. It is also desirable for the separator plate to have a large coefficient of dynamic friction which is related to shock and noise prevention as the plate is engaged. Designing for a large torque transmitting capacity, however, may result in a low coefficient of dynamic friction. U.S. Pat. No. 5,535,870 to Takezaki et al. discusses the difficulty in combining in one clutch the characteristics of high torque transmitting capacity, a large ratio of coefficient of dynamic friction to the coefficient of static friction and heat resistance. The '870 patent discloses the combination of two different kinds of commercially available friction members to realize the above characteristics. One friction member has a high coefficient of static friction while the other has a high coefficient of dynamic friction. The '870 patent, however, does not address, among other things, the issue of parasitic drag.
Parasitic drag occurs when the separator plate and the friction plates in a particular clutch pack assembly are not engaged. While in this condition, depending on the particular transmission design, the friction plates, the separator plates or both types of plates may be rotating independent of the other set. This condition is called freewheeling. In some freewheeling situations, the separator plates and the friction plates may be rotating in opposite directions. The plates are in a fluid bath which is used to cool the plates and to reduce wear as the plates are coupled and de-coupled. The fluid bath also carries away debris from the face of the plates. The fluid bath, while beneficial in several respects, does produce drag on the plates. For example, in a transmission where the fluid is transmission fluid, the rotation of the plates through the transmission fluid adjacent the plate is resisted by the transmission fluid. Consequently, energy must be expended to rotate the plates through the transmission fluid. Drag thus acts as a load on the engine in the freewheeling condition and results in, for example, reduced fuel efficiency. Inasmuch as the drag is thus undesired, it may be referred to as a parasitic force.
Additionally, as the plate rotates through the transmission fluid, some rotation of the transmission fluid is effected. Because the plates within a clutch pack assembly are very close to each other, the rotating transmission fluid acts upon the adjacent plates. If, according to the design of the particular transmission, the adjacent plates are moving in the opposite direction or are stopped, the resulting drag is yet another source of parasitic drag.
As discussed above, the presence of fluid between the separator plate and the friction plate in a clutch pack assembly may result in undesired rotation of plates due to drag. In addition to the deleterious effects on efficiency, the undesired rotation may result in an undesired effect referred to as "clunk". Clunk is most frequently noticed as a transmission is shifted out of neutral into reverse or a forward gear. As plates which are rotating in opposite directions are engaged and the rotation of one of the plates is suddenly reversed or stopped, the clutch pack assembly and associated components are mechanically shocked. This is manifested, in part, by a low frequency noise or clunk and a jolt to the vehicle. Clunk is thus undesired as it is an annoyance to the operator and subjects the transmission to undesired mechanical stresses.
U.S. Pat. No. 5,890,988 to Kasuya et al. discloses the use of a gear brake structure instead of a frictional engagement member such as a clutch pack assembly, to avoid rotation of equipment and thus to avoid the deleterious effects of drag. The '988 patent, however, is limited to eliminating drag caused by freewheeling of the frictional engagement members in the reverse gear only. Thus, parasitic drag produced by other frictional engagement members, such as in the first through fifth gears, is not reduced. Moreover, the gear brake of Kasuya et al. requires a significant alteration to the time proven design of transmissions which is realized only through expensive redesign efforts.
In addition to the shortcomings with respect to drag and clunk, prior art clutch packs suffer from a number of wear related shortcomings. One such shortcoming is the so called "break-in" period. Clutch components are designed to operate using friction between two surfaces initially rotating at different relative speeds in order to match the speed of rotation between the two surfaces. This results in the surfaces being worn down over time. When first placed into operation, there is normally an initial period where the change in characteristics such as drag, coefficient of dynamic friction and coefficient of static friction may be relatively rapid. This is commonly referred to as a "break-in" period. Rapid changes in transmission operation can be unsettling to an operator. Thus, it is desired to have a very short "break-in" period, if any, and a consistent rate of change throughout the remaining useful life of the clutch pack assembly such that any change is not perceived by the operator.
A characteristic which is closely related to the break-in characteristic is the shift characteristic. As the friction plate and separator plate are worn, the contact surfaces of components will vary as the components are worn. As the components wear, the contact area typically increases resulting in changes in the coefficient of friction. If the coefficient of friction increases, shifting may become jerky as the plates "grab" each other. Conversely, if the coefficient of friction becomes too small, shifting time may increase appreciably. Thus, the time required for disengagement of the lower gear and engagement of the higher gear may increase. Depending on the transmission, this may result in, for example, higher revolution rates of the lower gear or in noticeable hesitations in the acceleration. Rapid onset of shifting pattern changes may result in operator consternation while the same changes effected gradually over time are readily adapted to by the operator.
Notwithstanding the '988 patent which replaces an entire clutch pack assembly and the '870 patent which addresses the characteristics of both friction elements, improvements to shortcomings in prior art clutch pack assemblies have typically been directed to improvements in the friction plate. An exception to this is found in U.S. Pat. No. 5,048,654 issued to Yesnick. The '654 patent addresses the problem of increasing the coefficient of friction without increasing the wear of friction plates. Friction plate are typically made with a friction paper mounted on a steel plate. The friction paper is softer than steel, however, so increasing the roughness of the separator plate frequently results in undesired wear of the friction plate.
The '654 patent lists several modification processes aimed at modifying the surface of a separator plate which had not proven desirable. According to the '654 patent, modification was unsuccessfully attempted by various surface preparation procedures including sand blasting, shot peening and knurling. The '654 patent overcomes these process limitations with a separator plate formed entirely from ceramic material and a steel separator plate coated with a ceramic material to overcome the wear problems while increasing the coefficient of friction. As disclosed in the '654 patent, these improved plates exhibit an improved coefficient of friction without substantially increasing the wear of the friction plate lining. Ceramic plates, however, are more expensive to produce than steel plates and require extremely precise manufacturing processes.
While the '654 patent does not disclose when in the manufacturing process the unsuccessful texturing processes of sand blasting, shot peening and knurling were performed in the production of the separator plates, the texturing processes may be performed before or after a separator plate is formed from sheet steel. It is frequently more economical to texture the steel sheet prior to forming the separator plate, such as during cold rolling of the steel. Cold rolling is a process of forming a sheet of steel from stock material as is discussed, for example, in U.S. Pat. No. 5,025,547 to Sheu at al. In cold rolling, coil steel stock is processed through rolling mills, typically to achieve a material of the desired thickness and mechanical properties, by rolling the steel through a series of tandem rolls or work rolls, one on top of another, to reduce the thickness of the stock. The process is performed in conjunction with heating and then cooling of the stock to thereby harden the stock. To impart a texture to the steel, a textured set of rolls can be used. As the steel passes between the rolls, the texture of the rolls is pressed into the steel.
The textured rolls used in cold rolling process may be textured using the processes noted in the '654 patent. The final texture created with rolls textured by these methods, as well as the texture when the processes are performed after the forming of separator plates, is necessarily random in that each impingement of the roll is not individually controlled. Additionally, the texture so imparted tends to produce a topography with jagged spikes. Consequently, it is to be expected that as noted in the '654 patent, these procedures either did not enhance the frictional characteristics in the clutch pack or caused undue wear on the friction plates. There is needed, therefore, a process which imparts a texture resulting in an acceptable coefficient without unduly increasing the wear of the friction plate.
Another wear related shortcoming of prior art clutch packs is squawk. As the friction plate and separator plate are engaged, there will typically be a difference in the relative velocity of the two plates. As the two plates are forced together, torque is transferred from one spinning plate to the other plate until the two plates are rotating at the same speed. In some cases, there is an increase in the rate of torque transfer as the relative velocity of the plates approaches zero. This increase is commonly accompanied by a high pitched noise. This noise is known in the art as squawk. Obviously, squawk is distracting for the occupant(s) of the vehicle and consequently not desired.
The phenomena of squawk is not fully understood, however, the presence of fluid may reduce undesired squawk. As noted above, the fluid carries away debris which is created as the clutch separator plate and friction plate are worn down. Thus, perturbations to the rate of torque transfer which may result from the presence of unattached debris between the plates is reduced. Of course, as the amount of debris present in the fluid increases, the propensity for squawking also increases.
A better understood benefit of the fluid is the cooling it provides for the clutch pack assembly. This prevents uneven heating of the clutch friction plate and clutch separator plate which would otherwise reduce the life of the clutch pack. U.S. Pat. No. 5,682,971 to Takakura et al. discloses a friction plate having a pattern of grooves for passing oil from the inner portion of the friction plate to the outer portion of the friction plate. The grooves of the '971 patent further allow cooling of the friction plate while the two plates are mated. U.S. Pat. No. 5,878,860 discloses the use of a waved groove to accomplish a similar purpose. Neither of these patents, however, address the surface of the separator plate. Rather, as noted above, the patents are directed to the design of friction plates.
The above shortcomings of prior art separator plates are exacerbated by the tendency toward longer lived vehicles. For example, as a clutch pack ages, the amount of debris in the clutch pack increases. Thus, squawk and increased wear, both of the friction plate and the separator plate, increases. This problem is further exacerbated by a tendency toward maintenance-free vehicles as evidenced by vehicles not needing tune-ups before 100,000 miles. Thus, at least 100,000 miles of debris will accumulate in the transmission or other fluid before the fluid is changed. Consequently, increased robustness in overcoming the above discussed shortcomings is desired.
What is therefore desired is a separator plate which exhibits reduced parasitic drag. It is also desired that the separator plate reduce the occurrence of clunk and squawk. It is further desired that the separator plate exhibit an improved coefficient of friction without unduly increasing the wear of friction plates. It is preferred that the separator plate be simple and inexpensive to manufacture and have a longer life than the separator plates of the prior art.